Change name compute_sed to compute_L_lambda

[Danieljl96]

To let the user know beforehand the units of the spectrum.

[PabloCorcho]

A couple of things about this:

• This naming convention would not inform about the units, but the physical dimensions (i.e. flux density per wavelength or frequency unit) of the SED.
• In case it is implemented, I would recommend to only use lower-case letters (compute_l_lambda) as that is the PEP8 standard.
• The output SED is a Quantity, so I do not see any problem in leaving this method as it is.

[paranoya]

As noted in #8, we should implement a compute_photometry method (or compute_flux_densities, as called by @Danieljl96; in any case, we should use the same name in both classes) that is equivalent to compute_SED (or compute_L_lambda), using ssp.photometry instead of ssp.L_lambda.

Alternatively, we may modify compute SED to let the user choose between the spectral luminosity L_lambda for all wavelengths (the current behaviour), the spectral flux densities F_nu in the requested bands (orders of magnitude faster!), or both.

Regarding the naming convention, I must agree with @PabloCorcho, even though I find sed, f_nu, and especially l_lambda a little bit weird...